In the coming year we plan to extend our studies on substances other than divalent cations which may affect PTH secretion. In particular we will continue investigations of the effects of vitamin D and its various metabolites on hormone secretion; studies which suggest a physiologic feedback control between the major calcium regulating hormones. For these studies we will utilize the canine model we have developed for direct in vivo assessment of PTH secretion. In addition, we will study the effects of gastrointestinal and pituitary hormones on PTH secretion. Our long term studies of the mechanisms involved in the genesis and development of renal osteodystrophy will be continued.